The invention relates generally to an imaging system, and more specifically, to a phase shifting interferometry system used for improved measurements of surface topographies.
Phase shifting interferometry is a method for measuring surface topographies. It is based on a principle of phase shifting and uses wave nature of light to map variations in surface topographies. Phase shifting interferometry is a technique that determines shape of a surface by calculating a phase map from measured light intensities. A phase shifting interferometer typically includes an optical system, an electronic imaging system, a computer based signal processor, and a system for introducing a reference phase shift. This method typically involves electronic storage of light intensity patterns from a test surface measured for a sequence of three to five reference phase shifts. The variations in the light intensity patterns are then analyzed by the computer based signal processor as a function of phase shift to recover a profile of the test surface.
Typically, conventional phase shifting interferometers are capable of measuring a surface topography with a resolution on the order of one-thousandth of the wavelength of light used by the systems. However, when measuring surface topographies for very steep surfaces with a surface depth of the order of 10 μm or less, vibrations occurring in a phase shifting interferometer during a data acquisition cycle can distort measurements being recorded and lead to significant errors.
As a result, such highly precise instruments are typically installed in a fixed location or laboratory. Measurements from large parts or in-process parts on a factory floor or similar location are made indirectly, such as by taking a casting of a part surface, then transporting the casting to the fixed interferometer. The inability to use phase shifting interferometers in more flexible ways has significantly limited their use, and increased the cost and time associated with surface inspection for such large or difficult-to-transport manufactured articles.
Consequently, an improved phase shifting interferometer is needed. In particular, there is a need for a hand held or at least more transportable interferometer with improved image acquisition and image processing capabilities, and that can avoid or resolve errors induced due to the operating environment of the interferometer, particularly those due to vibration.